Today's computing devices generally use multiple types of memory systems. One type of memory system is so-called “main memory”, which comprises semiconductor memory devices that can be randomly written to and read from with very fast access times. As such, main memory is commonly referred to as random access memory (RAM).
Since semiconductor memory devices are relatively expensive, other higher density and lower cost memory systems are often used to complement main memory. One such example is a magnetic disk storage system (also called a “hard disk”). Magnetic disk storage systems are used for storing large quantities of data which can then be sequentially read into main memory as needed. However, while data can be stored more cheaply in a hard disk than in RAM, access times are longer, being generally on the order of tens of milliseconds for a hard disk compared to only hundreds of nanoseconds for RAM.
Yet another type of memory system is a so-called solid state disk (also called “solid state drive”, or “SSD”). An SSD is a data storage device that uses memory chips to store data, instead of the spinning platters found in conventional hard disks. SSDs are quite versatile and in fact the term “SSD” can be used to refer to two different kinds of products.
The first type of SSD is based on fast, volatile memory such as, for example, Synchronous Dynamic Random Access Memory (SDRAM), and thus is categorized by very fast data access times. Since such an SSD uses volatile memory, it typically incorporates internal battery and backup disk systems to ensure data persistence. Thus, if power is lost for whatever reason, the battery keeps the unit powered long enough to copy all data from SDRAM to a backup hard disk. Upon the restoration of power, data is copied back from the backup hard disk to SDRAM and the SSD resumes normal operation. This type of SSD is especially useful in accelerating applications that would otherwise be held back by the inherent latency of disk drives.
The second type of SSD uses non-volatile memory, such as, for example, Flash Electrically Erasable Programmable Read Only memory (EEPROM), to store data. Products incorporating this second type of SSD can have the same form factor as conventional mass storage products, and are typically used as low power, rugged replacements for hard disks. To avoid confusion with SSDs of the first type, SSDs of the second type are generally referred to as “Flash” SSDs. The remainder of the present disclosure is directed to Flash SSDs.
A conventional Flash SSD can consist of several non-volatile NAND-type Flash EEPROM semiconductor integrated circuits with a form factor corresponding to a 3.5″, 2.5″ or 1.8″ hard disk drive (HDD). The Flash SSD is connected to a host controller through conventional PATA or SATA interfaces, as with the HDD it was designed to replace. Therefore, Flash SSDs are limited by the available number of PATA or SATA connectors inside the computing device. As a result, the memory capacity provided by conventional Flash SSDs may be inadequate to meet the ever growing need for mass data storage brought on by current and future computing applications.
Against this background, there is clearly a need in the industry for an improved mass data storage system that uses non-volatile memory.